1. Field of the Invention
The present invention relates to a process for producing a powder material of lead perovskite ceramics and, more particularly, to a process for producing a powder material for niobium-containing lead perovskite ceramics which has a uniform and very small particle diameter and a high bulk density, which is easy to sinter, and which is capable of producing functional ceramics suitable for use as a piezoelectric element, semiconductor, sensor, optoelectronics material, dielectric, in particular, dielectric of a multilayer capacitor, and the like.
2. Description of the Prior Art
Lead perovskite ceramics are a new material which is not only widely used as a piezoelectric element but also whose use as a dielectric of a multilayer capacitor or the like has been extensively investigated in recent years.
It is for the following reasons that such new uses for lead perovskite ceramics have been investigated:
(1) Since lead perovskite ceramics have a high dielectric constant and a larger dielectric capacity per unit volume than a conventionally used dielectric, it is possible to reduce the size and increase the capacity of a capacitor.
(2) The sintering temperature of lead perovskite ceramics, which is not higher than 1,200.degree. C., is lower than the sintering temperature of barium titanate conventionally used, which is 1,300.degree. to 1,400.degree. C. Thus it is possible to use silver, which is cheaper than platinum or palladium conventionally used and has a comparatively lower melting point, as an internal electrode material.
(3) Since many kinds of materials can constitute a perovskite phase with lead, it is possible to produce many kinds of dielectric materials which correspond to various uses by appropriately changing the composition.
As a process for producing a powder material for such lead perovskite ceramics, a dry process and a wet process are conventionally known.
The dry process is a method of producing lead perovskite ceramics by mixing the oxide powders of the components which constitute a perovskite phase in accordance with the composition and calcining the mixture.
In contrast, the wet process conventionally adopted is a method of producing lead perovskite ceramics by preparing a mixed solution of all the components which constitute a perovskite, adding a precipitate-forming liquid such as an alkali to the mixed solution for effecting coprecipitation, and separating out, drying and calcining the precipitate obtained.
By the above-described dry process, however, it is difficult to obtain a powder material having a uniform composition, and the powder material obtained contains many pyrochlore phases whose dielectric constant is disadvantageously low. In order to reduce the pyrochlore phase and increase the perovskite phase which exhibits ferroelectricity, it is necessary to raise the calcination temperature. When the calcination temperature is high, however, the powder material becomes coarse, leading to a new problem in that it is difficult to obtain a powder material easy to sinter.
In contrast, it is comparatively easy to obtain a powder having a uniform composition by the above-described wet process. However, since it has a uniform composition, the particles are apt to coagulate to form secondary particles during the formation of precipitate, drying or calcination. The particle diameter therefore becomes large, so that it is difficult to obtain a powder material easy to sinter in the same was as in the case of raising the calcining temperature in the dry process.
In addition, in the wet process utilizing coprecipitation, since a precipitate-forming liquid in common with each component, namely, having the same concentration is used, when the precipitate-forming abilities of the components are different from each other, it is difficult to obtain a perovskite having a desired composition.
Furthermore, since the grain size of a ceramic obtained by sintering a powder material by a conventional dry or wet process is generally not less than 5 .mu.m, such a ceramic is not adaptable to an element which is required to be reduced in size and increased in capacity such as a multilayer capacitor which is often restricted in the thickness of a dielectric layer.
As a result of studies on niobium-containing lead perovskite ceramics undertaken by the present inventors, the following facts have been found.
As a powder material for functional ceramics, a powder having a uniform composition is preferable.
However, from a wrong judgement that a uniform composition is obtained by uniformly mixing the respective compounds of all components, efforts have been made in the prior art to uniformly mix all the components of a perovskite in order to obtain a powder material having a uniform composition without considering the difference in the behavior in a solid phase reaction between components.
However, lead is very reactive and has a very high activity with niobium in comparison with a metal element such as cobalt and nickel. Therefore, if all the components are uniformly mixed as in the prior art in a solid phase reaction which is carried out at a comparatively low temperature in order to prevent the powder material from becoming coarse, the reaction between lead and niobium takes precedence over the reaction between lead and other metals.
This is the reason why the production of the perovskite phases which exhibits ferroelectricity is insufficient in the prior art while many pyrochlore phases, which are a compound of lead and niobium, are produced.
As a countermeasure, a method of preventing the generation of a pyrochlore phase by calcining a mixture of an oxide of A and an oxide of Nb at a temperature of 900.degree. to 1,000.degree. C. to prepare a niobiate and reacting PbO with the niobiate is proposed (Mat. Res. Bull. Vol. 17, 1245 to 1250, 1982, S. L. Swarts and T. R. Shrout).
By this method, however, although pyrochlore phases are greatly reduced, since the temperature for preparing the niobiate of A is high, pulverization is difficult, and it is difficult to obtain fine perovskite having a particle size of not more than 0.5 .mu.m.